1. Field of the Invention
This invention relates generally to a mechanical seal for sealing a rotatable shaft in the housing of a fluid machine against leakage of fluids along the shaft. More particularly, it relates to a controllable mechanical seal wherein the thickness of a thin lubricating fluid film separating two seal face elements is controlled by an externally applied voltage.
2. Description of the Prior Art
In order to assist in a better understanding of the invention, its operation and its advantages, it is believed that a preliminary discussion of mechanical seals in general is in order. As will be appreciated by those skilled in the art, a mechanical seal is formed with two face elements. One element is attached to the housing of the machine to be sealed, and the other is attached to and rotates with the shaft. One of the elements is fixed so that no movement occurs axially relative to the shaft. It is referred to as the fixed face element. The other is movable axially along the shaft, and is referred to as the floating face element. The face elements are located in opposed relationship to each other, and are arranged so that in response to fluid pressure, spring pressure or both, a sealing relationship will be obtained between them to prevent leakage out along the shaft.
It has been found that a successful mechanical seal is obtained not when the elements are in direct physical contact with each other, but rather when a thin lubricating fluid film is provided between their opposing face surfaces during steady state operation. This fluid film prevents or reduces wear due to direct mechanical contact of the elements, thereby avoiding the possibility of mechanical damage or failure of the seal. However, the thickness of the fluid film must not be too large, as this will cause excessive leakage.
Further, it has been found that the thickness of the fluid film is determined by the precise geometry of the face surfaces. In particular, if the two face surfaces are perfectly flat and parallel, such that a uniform gap is formed between them, the floating element will move into physical contact with the fixed element. The fluid film will collapse, resulting in a zero film thickness. In order to maintain a finite film thickness so as to prevent such a collapse, a sufficiently large opening force, which tends to move the floating element away from the fixed element, must be generated by fluid pressure within the gap. For this to happen, the opposing surfaces must not be parallel, but rather must converge radially in the direction from the high pressure side to the low pressure side of the seal. If this convergence is increased, the opening force will increase and the film thickness will be increased. Similarly, if the convergence is decreased, the film thickness will be decreased.
Generally, mechanical seals are manufactured with the face elements designed such that their face surfaces have a predetermined convergence during steady state operation of the machine, taking into account the anticipated thermal and mechanical deformations of the elements. Thus a reasonable film thickness is realized. It should be understood that the thickness of film is relatively small, on the order of approximately 50-200 microinches. The deformations are equally small, on the order of approximately 20-100 microinches. Heretofore, conventional mechanical seals have been designed and built very carefully based upon all the anticipated deformations so as to produce an acceptable film thickness at the condition of steady state operation. The film thickness was dependent upon the seal design characteristics, such as type of material, configuration, etc., and operating conditions such as temperature, pressure, speed, load and fluid characteristics. Thus the film thickness could not be controlled once the seal had been placed into service. Accordingly, conventional seals would experience face damage and wear when a wide range of operating conditions, including transients, was encountered.
It would therefore be desirable to provide a controllable mechanical seal in which the thickness of a thin fluid film separating two seal face elements could be controlled by external means. If this controllable feature could be provided, the thickness of the film could be varied in response to changes in the operating conditions so as to maintain an optimum film thickness for a wide range of such conditions.
In U.S. Pat. No. 3,433,489, issued to W. J. Wiese on Mar. 18, 1969 and assigned to the assignee of this application, there is disclosed a mechanical seal assembly which includes a collector ring formed in one of the sealing face elements. Fluid flowing past the elements is collected in the collector ring and flows into a pressure chamber so as to produce a fluid pressure under certain operating conditions. This fluid pressure is applied to the element having the pressure chamber in an effort to control leakage past the elements.
In U.S. Pat. 3,948,530, issued to E. J. Gyory on Apr. 6, 1976 and also assigned to the assignee of this application, there is disclosed an externally adjustable mechanical seal in which hydrostatic pressure is applied to a pair of axially spaced pressure areas on the external surface of a stationary sealing element. The pressure areas communicate with pressure passages which are appropriately valved to provide settings for high pressure, low pressure or atmospheric pressure to be communicated with one or both of the pressure areas. As a result, the element is deformed to provide full surface contact, internal diameter surface contact, or external diameter surface contact of the sealing elements.
U.S. Pat. No. 4,434,987, issued to Albers et al on Mar. 6, 1984, discloses a seal device which includes a seal ring rotatable with a shaft. This rotating seal ring is surrounded on both of its radially extending surfaces and on its circumferentially extending surface by a non-rotatable but slidable seal ring connected to a housing. Sensors are connected to the non-rotatable seal ring for sensing the width of each radial plane gap. Control means is responsive to the sensors for activating an electromagnet which develops a magnetic force which slides the non-rotatable seal ring in order to maintain a selective width for the radial plane gaps.
There remains a need in the art for a controllable mechanical seal wherein the thickness of a thin lubricating fluid film separating two seal face elements is controlled by an externally developed force which is applied to at least one of the elements so as to cause deformation of its face surface. By controlling this deformation, the convergence of the opposing face surfaces is adjustable. The opening force is thus controllable such that an optimum film thickness may be obtained for a wide range of operating conditions.